Various instruments have found use for applications requiring level sensing of, for example, a liquid or bulk material in a vessel. One such instrument comprises a capacitance-type level sensor. With any such instrument it is desirable to ensure safety of both equipment and personnel.
With a capacitance-type level sensor the process instrument includes a probe assembly and an electric housing assembly. The probe assembly includes an adaptor nut receivable in an opening of a vessel and a probe extending through the adaptor nut into the process vessel. The probe exhibits capacitance. Capacitance increases as part of the probe is surrounded by material of dielectric constant greater than one. The capacitance present between the probe and the vessel can be measured to determine level of the material in the vessel.
To measure capacitance between the probe and the vessel the probe must be insulated from the adaptor nut. Also, the vessel interior is typically sealed from outside, particularly in high temperature and/or pressure applications. The sealing and insulation is often provided by a plastic bushing surrounding the probe, where it is held to the adaptor nut.
In the event that a fire occurs, concerns exist that the plastic seal being a "soft seal" might fail. Failure of the seal could cause spewing of flammable material from the process vessel to feed the fire. Advantageously, the process instrument would prevent product from feeding the fire.
The present invention is intended to satisfy one or more of the problems discussed above.